I just had FTTP installed. I also have a set of TP-Link Powerline adapters
Unfortunately, the modem is in the corridor where I don't have much space for a router.
I was wondering whether, it is possible to plug the Power-line adapter directly into the Openreach modem (via ethernet) and use that to connect to other rooms in the flat, so I wouldn't need a router at all.
Would this work?
You still need a router. You can use the power line adaptors to connect the ONT to the router.
The only way it would work without a router is to connect a single PC to the power line adaptor and configure a PPPoE session. You would not be able to use WiFi.